US 2006/0144155 A1 discloses a flow sensor system comprising upstream and downstream pressure sensing devices for detecting a differential pressure in a flow. Each pressure sensing device comprises a diaphragm, a capacitor and an inductor to form an LC tank circuit. The capacitor can be mechanically coupled to the diaphragm such that a deflection of the diaphragm in response to fluid pressure applied thereto causes a change in the resonance frequency of the LC tank circuit.
A problem of the sensor system disclosed in US 2006/0144155 A1 is that it is not capable of measuring flow rate in the instance of non-steady fluid flow conditions, i.e. fluid flow that is explicitly depending on time. Namely, in US 2006/0144155 A1 flow rate is determined by substituting the differential pressure as measured in the well known Bernoulli equation for incompressible liquid flow. The latter equation does hold for spatially varying fluid flow, however, it fails to hold true for time varying i.e. non-steady fluid flow.